1. Field of the Invention
This invention relates generally to alloys which are characterized by high temperature strength characteristics, and more particularly to alloys of the Co-Cr-C type.
2. Description of the Prior Art
Unidirectional solidification of lamellar eutectic alloys is known in the art, as indicated in Kraft U.S. Pat. 3,124,452, or Giessereforschung 24 (1972) pp. 45-53, for instance. That technique refers to a process in which the alloy is partially melted to form a liqud-solid interface. The interface is caused to be moved in a unidirectional fashion as the alloy is cooled through an appropriate transformation temperature. In this way the crystallites of each phase grow or form normal to the interface or solidification front between the solid and molten metal, i.e. in parallel to the direction in which the solidification front moves relatively to and through the alloy if the solidification front is at least substantially planar, which can be achieved by means of known cooling devices of appropriate kind. The conditions of unidirectional solidification can be determined by the following equation in which the ratio of temperature gradient G and growth rate v is determined by ##EQU1## WHEREIN M IS THE SLOPE OF THE LIQUIDUS LINE AT THE POINT OF DEVIATION FROM EUTECTIC COMPOSITION, D is the diffusion coefficient of the liquid atoms, .DELTA.c is the deviation of the melt composition and K.sub.i C.sub.i is a constant governed by the impurity concentration C.sub.i. Fulfillment of this condition serves to prevent the formation of dendrites or cell boundaries.
The melt is convectionless with no thermal fluctuations at the interface, in order to prevent the formation of growth bands or other growth defects.
This technique has been applied to Co-Cr-C alloys in the prior art. For instance, Thompson, U.S. Pat. No. 3,564,940, and German publication No. 1,928,258 report an alloy of the composition 35-45% wt Cr, 2.2-2.5% wt C and 52.4 - 62.8% wt Co. The Co-Cr-C alloys of the Thompson patent are aligned polyphase structures which solidify according to the monovariant eutectic structure: at a fixed pressure these compositions are monovariant thermodynamically and involve, in ternary systems for example, the three phase equilibrium between the melt and two solids over a temperature and composition range and not, as in the binary or pseudo-binary systems, at a fixed temperature and composition. Those compositions are located on a eutectic trough.
However, the alloys reported in Thompson are generally characterized by quite unsatisfactory strength characteristics, particularly time-dependent creep strength, and, therefore, the range of application of such alloys is limited. Although that prior patent indicates that its composition had a contemplated utility in the formation of gas turbine blades, in practice, those alloys have not been found to be sufficiently satisfactory.
Efforts had thus been made to improve the strength properties of this system of alloys, without success. For instance, methods are known for improving the high temperature strength characteristics, but not without sacrifice of other properties, such as corrosion resistance.
Lemkey et al U.S. Pat. No. 3,552,953, disclose another Co-Cr-C alloy of the composition 45.2 - 49.2 % wt Co, 49 - 53 % wt Cr and about 1.87 % wt C which are unidirectionally solidified such that a carbide of the form Cr.sub.23 C.sub.6 is dispersed in a skeletal distribution in the matrix. This type of alloy, however, is quite different from the alloy of the present invention.
A need continues to exist for a technique of improving the high temperature strength characteristics of Co-Cr-C alloys, without sacrifice of other desirable properties of the alloy, particularly without sacrifice of corrosion resistance.